Wheat storage proteins are classified on the basis of their solubility into two classes. The gliadins are readily soluble in aqueous alcohols and are monomeric proteins with only intramolecular disulphide bonds. The glutenins are present in high molecular weight polymers, stabilised by intermolecular disulfide bonds and are not soluble in aqueous alcohols without reducing agent (Kasarda 1989). These proteins are present in high amount in the endosperm and are considered to act as a store of nitrogen, carbon and sulphur for seed germination.
Glutenins form a continuous proteinaceous network called gluten. The unique physico-chemical properties of gluten determine the ability of wheat dough to be processed into baked goods (bread, biscuits, cakes), pasta noodles and other food products. It is understood that the glutenins, which form crosslinks with each other through disulfide bonds, are the most important molecules producing the viscoelastic properties of wheat flour dough (MacRitchie 1992). The unique position of wheat in bread making is due to the ability of the dough to retain gas on expansion. The gluten accounts for about 10% of the dough, and consists mainly of proteins (70-80%) together with starch and lipids. Starch could be granular and damaged starch. The lipid reserves of wheat are non-polar, structural and endosperm lipids (Gan et al., 1995). Structural lipids are also called polar lipids. The endosperm lipids are divided into non-starch lipids and starch lysophospholipids. The structure and properties of gluten are determined by molecular interactions and it is important that these be understood if the functional properties of gluten are to be manipulated.
A dough results from a large variety of interactions between flour constituents facilitated by water. Starch takes up about 46% of the water and damaged starch contributes significantly to the water absorption. It has been shown that during hydration, proteins exude visible strands or fibrils. Specific proteins of flour are bound to flour lipids (polar) upon addition of water (Morrison 1989).
Dough development is visualised as a re-orientation of glutenin polymers to form a membrane network with viscoelasticity and gas retaining properties. Covalent (disulfide) and noncovalent (hydrogen, hydrophobic and ionic) bonds are involved in formation of a fully developed dough. Interactions are further modified during fermentation, baking and even after baking. The disulphide bonds of flour proteins play a key role in the interactions in dough. The bonds form relatively strong crosslinks within and between polypeptide chains and also stabilise other less energetic bonds. Disulphide bonds provide the required stability for the protein matrix until the loaf structure is set by the gelatinisation of the starch and the thermal denaturation of the proteins during baking. Hydrogen bonds are considerably weaker than covalent bonds, but contribute significantly to the structure of dough. A unique feature is the ability to interchange with other hydrogen bonds, which facilitate reorientation of protein chains and allow for stress relaxation. Hydrophobic bonds result from nonpolar groups of flour constituents. Because these bonds are reversible, they can readily accommodate viscous flow and thereby facilitate mechanical dough development. Ionic bonds play relatively small part in dough structure formation but some specific components have an ionisable part or parts. Therefore ionic bond interactions could be important for the Theological properties (for a review, see Bushuk 1998).
A major limitation to evaluating the contributions of various groups of proteins, and of specific structural features of these molecules, to dough functionality has been the lack of appropriate systems that allow specific proteins to be incorporated and tested within the dough. The situation has recently changed, however, due to two advances. The first is the development of small scale testing equipment (Mixograph, Extensograph) with appropriate procedures for incorporating exogenous proteins, including polymeric glutenins into the dough (Bekes et al., 1994). Advantages of this system are the small amount of proteins required for test and the ability to rapidly test multiple samples produced by, for example, protein engineering. The second recent advance is the development of a reliable transformation system for wheat (Weeks et al., 1993, Witrzens et al., 1998), which allows the modification of storage protein composition by the expression of new proteins with, for instance, designed characteristics.
To alter protein-protein, protein-lipid and protein-starch interactions within the gluten matrix, the present inventors have developed a system which enables the incorporation of new surface active molecules or parts of molecules into the gluten matrix.